Photodiode device monolithically integrating waveguide element with photodiode element type of optical waveguide

ABSTRACT

A photodiode (PD) device that monolithically integrates a PD element with a waveguide element is disclosed. The PD device includes a conducting layer with a first region and a second region next to the first region, where the PD element exists in the first region, while, the waveguide element exists in the second region and optically couples with the PD element. The waveguide element includes a core layer and a cladding layer on the conducting layer, which forms an optical confinement structure. The PD element includes an absorption layer on the conducting layer and a p-type cladding layer on the absorption layer, which form another optical confinement structure. The absorption layer has a length at least 12 μm measured from the interface against the core layer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of Japanese Priority Applicationsof JP2018-037296 filed on Mar. 2, 2018 and JP2018-090558 filed on May 9,2018, which are incorporated herein as references.

BACKGROUND 1. Field of Invention

The present invention relates to a photodiode with a type of opticalwaveguide.

2. Related Background Arts

A Japanese Patent Application laid open No. JP2013-110207A has discloseda photodiode device that monolithically integrates a waveguide elementand a photodiode element with a type of optical waveguide opticallycoupled with the optical element on a substrate common to the waveguideelement and the photodiode element.

An optical communication system currently installed often configures,what is called, the wavelength division multiplexing (WDM) usingwavelengths of 1528 to 1565 nm, which is often called as the C-band inthe field. As the capacity of data to be transmitted explosivelyincreases, an optical communication system using longer wavelengths,specifically, 1565 to 1612 nm, which is called as the L-band, has beeninvestigate. However, a PD commercially available in the field oftenshows large temperature dependence, especially in optical sensitivitythereof, in the L-band. For instance, the optical sensitivity of a PDwidely degrades at a temperature lower than 0° C. Accordingly, anadvanced PD applicable to the L-band at a low temperature has beendemanded.

Also, a future optical communication system inevitably requests a highspeed operation exceeding 400 Gb/s and a complex modulationconfiguration such as multi-level amplitude modulation combined with aphase modulation. Accordingly, a photodiode applicable such high speedand high degree of optical modulation will be inevitably requested to beoperable in higher frequencies. A waveguide PD is determined infrequency response thereof primarily by a time constant caused byparasitic and inherent capacitance and resistance attributed tostructures around an absorption layer thereof and a carrier transporttime within the absorption layer. In order to enhance the frequencyresponse, both the carrier transport time and the time constant arepreferably shorter. However, a thinned absorption layer to shorten thecarrier transport time inevitably increases the capacitance thereof toincrease the time constant. Accordingly, the field has also requested tomake above two subjects consistent to each other.

SUMMARY

An aspect of the present invention relates to a photodiode device thatintegrates a waveguide element with a photodiode element type having awaveguide structure. The photodiode device includes a substrate, thephotodiode element and the waveguide element. The substrate provides aconducting layer doped with n-type impurities, the conducting layerhaving a first region and a second region next to the first region. Thewaveguide element, which is provided in the first region of theconducting layer, includes a core layer on the conducting layer and acladding layer on the core layer. The photodiode element, which isprovided in the second region of the conducting layer and opticallycoupled with the waveguide element, includes an absorption layerprovided on the conducting layer and having a bandgap wavelength longerthan 1612 nm, where the absorption layer abuts against the core layer inthe guide element, and a p-type cladding layer provided on theabsorption layer and doped with p-type impurities. A feature of thephotodiode device of the present invention is that the absorption layerin the photodiode element has a length at least 12 μm along an opticalaxis thereof measured from an interface against the core layer in thewaveguide element.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other purposes, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a plan view showing an optical device providing a photodiode(PD) unit that monolithically integrates PD elements with waveguideelements on a substrate;

FIG. 2 is a cross sectional view of the PD device taken along the lineII-II indicated in FIG. 1;

FIG. 3 magnifies a portion of a PD element shown in FIG. 2;

FIG. 4 is a cross sectional view of a PD element and a waveguide elementthat are taken along the line IV-IV indicated in FIG. 1;

FIG. 5 is a plan view magnifying an optical coupler monolithicallyintegrated in the PD device;

FIG. 6 shows differences in output power between an in-phase componentand a quadrature component in the signal components each output from theoptical coupler shown in FIG. 5 designed for the L-band;

FIG. 7 shows differences in the output power between the in-phasecomponents and the quadrature phase components of the signal componentsoutput from the optical coupler designed for the C-band;

FIG. 8 schematically shows wavelength dependence of sensitivity of agenerally available PD element;

FIG. 9 shows normalized power distribution in the absorption layer of aPD element at a wavelength in the C-band;

FIG. 10 shows the normalized power distribution in the absorption layerof a PD element at a wavelength in the L-band;

FIG. 11 shows wavelength dependence of normalized sensitivity of a PDelement of an embodiment of the present invention, where the PD elementhas a length of 12 μm along an optical axis thereof;

FIG. 12 is a cross sectional view of another PD element and anotherwaveguide element that are modified from the PD element and thewaveguide element shown in FIG. 4;

FIG. 13 is a cross sectional view of still another PD element, where thestill another PD element is modified from the PD element shown in FIG.4; and

FIG. 14 is a cross section view of a PD element and a waveguide elementaccording to a conventional arrangement.

DETAILED DESCRIPTION

Next, some embodiments according to the present invention will bedescribed referring to drawings. The present invention is not restrictedto those embodiments and has a scope defined in claims attached heretobut may cover all changes and modifications for element in the claimsand equivalents thereto. In the description of the drawings, numeralsand/or symbols same with or similar to each other will refer to elementssame with or similar to each other without duplicating explanations.Also, a term “un-dope” in the specification means that a semiconductormaterial is not intentionally doped and has impurity density smallerthan, for instance, 1×10¹⁵ cm⁻³.

As shown in FIG. 1, the optical device 1A includes a photodiode device2A and amplifying units, 3A and 3B. The PD device 2A includes waveguideelements formed on a substrate made of semiconductor material, typicallyindium phosphide (InP), having a rectangular slab shape. The PD device2A provides two input ports, 4 a and 4 b, an optical coupler 5, four PDelements, 6 a to 6 d, and four capacitor elements, 7 a to 7 d, all ofwhich are integrally formed on the semiconductor substrate.

The PD device 2A in the rectangular slab thereof has a pair of edges, 2a and 2 b, where the former edge 2 a provides the input ports, 4 a and 4b. One of the input ports 4 a optically couples with an external fiberto receive an optical signal L_(a) that multiplexes four signalcomponents, L_(C1) to L_(C4), configured with the quadrature phase shiftkeying (QPSK). The signal components, L_(C1) to L_(C4), are multiplexedin a wavelength band of 1565 to 1612 nm, which corresponds to the C-banddefined in the international telecommunication union telecommunicationstandardization (ITU-T). The other input port 4 b receives a localsignal L_(b). Two input ports, 4 a and 4 b, optically couple with theoptical coupler 5 through waveguide elements, 8 a and 8 b, where thewaveguide elements, 8 a and 8 b, and other waveguide elements, 8 c to 8f, are formed by a core layer and two or more cladding layers eachhaving refractive indices smaller than that of the core layer andsurrounding the core layer.

The optical coupler 5, which includes multi-mode interference (MMI)couplers, configures an optical 90° hybrid to extract the four signalcomponents, L_(C1) to L_(C4), contained in the optical signal L_(a) byperforming interference between the optical signal L_(a) and the localsignal L_(b). Former two signal components, L_(C1) and L_(C2), havephases difference by π from each other, while, latter two signalcomponents, L_(C2) and L_(C4), also have phases difference by π fromeach other but different from 90° from corresponding to the formersignal components, L_(C1) and L_(C2). That is, the former two signalcomponents, L_(C1) and L_(C2), are often called as the in-phasecomponent, while, the latter two signal components, L_(C2) and L_(C4),are called as the quadrature component.

FIG. 5 is a plan view magnifying the optical coupler 5. As shown in FIG.5, the optical coupler 5 provides two MMI couplers, 51 and 52, where theformer coupler 51 has the configuration of two-inputs by four-outputs(2×4), while, the latter coupler 52 has the configuration of two-inputsby two-outputs (2×2). The 2×4 coupler 51 in one of input ports coupleswith the waveguide element 8 a, while, the other input port couples withthe waveguide element 8 b. Two of output ports of the first MMI coupler51 couples with two input ports of the other MMI coupler 52 throughwaveguide elements, 8 g and 8 h, with restive optical lengths differentfrom each other. That is, the waveguide element 8 h bends outwardly in aphase shifter 8 i so as to be apart from the other waveguide 8 g, whichsets the optical length of the waveguide element 8 h to be longer thanthe optical length of the other waveguide element 8 g. Thus, lightpropagating in the waveguide element 8 h is delayed in a phase thereofagainst the other light propagating in the waveguide element 8 g. Theother two output ports of the 2×4 MMI coupler 51 directly couple withthe waveguide elements, 8 c and 8 d. Also, two output ports of the 2×2MMI coupler 52 couple with the waveguide elements, 8 e and 8 f.

Referring to FIG. 1 to FIG. 4 again, the PD elements, 6 a to 6 d, whichhave an arrangement of a PIN photodiode, are disposed along the edge 2 bof the PD device 2A. The PD elements, 6 a to 6 d, each optically couplewith the four outputs of the optical coupler 5 through the respectivewaveguide elements, 8 c to 8 f. The PD elements, 6 a to 6 d, which aresupplied with constant biases in respective cathodes thereof, receivethe signal components, L_(C1) to L_(C4), from the optical coupler 5through the waveguide elements, 8 c to 8 f, and generate photocurrentscorresponding to the signal components, L_(C1) to L_(C4). The PD device2A provides signal pads, 21 a to 21 d, which are disposed along the edge2 b, each corresponding to the PD elements, 6 a to 6 d, and beingelectrically connected with anodes of the PD elements, 6 a to 6 d,through bonding wires, 20 a to 20 d.

The capacitor elements, 7 a to 7 d, are a type of metal-insulator-metal(MIM) capacitor providing a lower metal layer, an upper metal layer, andan insulating film 45 sandwiched by the lower and upper metal layers.The lower and upper metal layers are stacked metals of, for instance,titanium-tungsten and gold (TiW/Au), titanium, platinum, and gold(Ti/Pt/Au), and so on. The capacitor elements, 7 a to 7 d, which aredisposed in side by side with respect to the PD elements, 6 a to 6 d,along the edge 2 b, are connected between bias interconnections 42,which supply the biases to the PD elements, 6 a to 6 d, and the groundas the upper metal layer. The bias interconnections 42 are the lowermetal layers in the capacitor elements, 7 a to 7 d; while, the uppermetal layers 43 in the capacitor elements, 7 a to 7 d, are extracted toground pads, 23 a to 23 d, that are connected with a back metal providedin a back surface of the substrate 10 through substrate vias, which arenot illustrated in the figures.

The capacitor elements, 7 a to 7 d, in the respective lower metal layers42 extend to the bias pads, 22 a to 22 d; while, the upper metal layers43 extend to the ground pads, 23 a to 23 d, disposed closer to the edge2 b. Thus, the capacitor elements, 7 a to 7 d, are disposed between thebias pads, 22 a to 22 d, and the ground pads, 23 a to 23 d. The biaspads, 22 a to 22 d, receive the biases form the outside of the PD device2A through bonding wires, 20 i to 20 m. The ground pads, 23 a to 23 d,are connected with ground pads, 62 a, 62 c, 62 d, and 62 f, in theamplifying units, 3A and 3B, through bonding wires, 20 e to 20 h.

Thus, the PD device 2A of the present embodiment monolithicallyintegrates, on the substrate 10, the capacitor elements, 7 a to 7 d, andthe PD elements, 6 a to 6 d, where both elements are closely and denselyarranged along the edge 2 b of the substrate 10. Moreover, the uppermetal layers 43 of the capacitor elements, 7 a to 7 d, are grounded tothe back metal 50 of the substrate 10 through the substrate vias andconnected with the ground of the amplifying units, 3A and 3B, throughthe back metal 50, which may enhance and stabilize the ground around thePD device 2A.

The amplifying units, 3A and 3B, which are a type of trans-impedanceamplifier (TIA) that converts photocurrents generated by the PDelements, 6 a to 6 d, into respective voltage signals, are disposedbehind the PD device 2A. Specifically, the amplifying unit 3A, whichprovides two signal pads, 61 a and 61 b, differentially andcomplementarily coverts the photocurrents entering the signal pads, 61 aand 61 b, into a voltage signal; while, the other amplifying unit 3B,which also provides two signal pads, 61 c and 61 d, differentially andcomplementarily converts the photocurrents entering thereto into anothervoltage signal. The signal pads, 61 a to 61 d, are arranged along theedge 2 b of the PD device 2A and coupled with the signal pads, 21 a to21 d, in the PD device 2A through respective boding wires, 20 a to 20 d.

The amplifying unit 3A further provides three ground pads, 62 a to 62 c,arranged along the edge 2 b of the PD device 2A such that the former twoground pads, 62 a and 62 b, places the signal pad 61 a therebetween,while, the latter two ground pads, 62 b and 62 c, places the othersignal pad 61 b therebetween, where outer two ground pads, 62 a and 62c, are connected with the ground pads, 23 a and 23 b, in the PD device2A through bonding wires, 20 e and 20 f, but the center ground pad 62 bis not directly coupled with the PD device 2A. Also, the otheramplifying unit 3B provides three ground pads, 62 d to 62 f, arrangedalong the edge 2 b such that the former two ground pads, 62 d and 62 e,put the signal pad 61 c therebetween, while, the latter two ground pads,62 e and 62 f, put the other signal pad 61 d therebetween, where theouter two ground pads, 62 d and 62 f, are coupled with the ground pads,23 c and 23 d, in the PD device 2A through respective bonding wires, 20g and 20 h, but the center ground pad 62 e is not directly coupled withthe PD device 2A.

FIG. 2 is a cross sectional view of two PD elements, 6 c and 6 d, andFIG. 3 magnifies the PD element 6 d. Other PD elements, 6 a to 6 c, havearrangements same with those shown in FIG. 3. FIG. 4 magnifies a portionthat couples the PD element 6 d with one of the waveguide elements 8 d,while, other PD elements, 6 a to 6 c, and the waveguide elements, 8 a to8 c, have same arrangements with those shown in FIG. 4. As shown in FIG.4, the PD element 6 d and the waveguide element 8 f are monolithicallyintegrated on a substrate 10 that may be made of semi-insulating indiumphosphide (InP).

As shown in FIG. 3, the PD element 6 d includes, on the substrate 10, anconducting layer 11 highly doped with n-type impurities and a PDstructure 19A with a type of waveguide in a region D of the conductinglayer 11. The PD structure 19A includes, on the conducting layer 11, anabsorption layer 13, a p-type cladding layer 14 on the absorption layer13, and a p-type contact layer 15 on the p-type cladding layer 14. ThePD elements 6 d further provides a buffer layer 12 between theconducting layer 11 and the absorption layer 13. Thus, the conductinglayer 11, the p-type cladding layer 14, and the p-type contact layer 13are the first to third layers in the present embodiment, while, thebuffer layer 12 is the fourth layer in the present embodiment.

The conducting layer 11 may be made of InP doped with silicon (Si) bydensity greater than, for instance, 1×10¹⁷ cm⁻³ and have a thickness of1 to 2 μm. The buffer layer 12 may be an n-type layer doped with n-typeimpurities by density smaller than that in the conducting layer 11, forinstance, the density smaller than 1×10¹⁶ cm⁻³ when the impurity is Si,or in an alternative, the buffer layer is an intrinsic type (i-type)layer intentionally doped with no impurities. The buffer layer 12 hasbandgap energy greater than that of the absorption layer 13 but smallerthan, or equal to, the bandgap energy of the conducting layer 11. In anexample, the buffer layer 12 is Si-doped InP with a thickness of 0.1 to0.3 μm.

The absorption layer 13 may be made of In_(x)Ga_(1−x)As (0<x<1) with abandgap wavelength longer than 1612 nm, for instance, 1650 nm, where abandgap wavelength corresponds to an inverse of the bandgap energy,which means that the absorption 13 layer shows sensitivity in theL-band. Specifically, the absorption layer 13 may be an undoped InGaAs,or an n-type InGaAs doped with Si by density less than 3×10¹⁶ cm⁻³. Theabsorption layer 13 has a thickness of 0.1 to 0.4 μm, preferably, athickness of 0.2 to 0.3 μm. The p-type cladding layer 14 is made ofp-type InP doped with zinc (Zn) by density greater than 2×10¹⁷ cm⁻³ andhas a thickness of 1.0 to 2.5 μm. The p-type contact layer 15 may bemade of InGaAs also doped with Zn by density greater than 1×10¹⁸ cm⁻³and has a thickness of 0.1 to 0.3 μm.

The buffer layer 12, the absorption layer 13, the p-type cladding layer14, and the p-type contact layer 15 collectively form a mesa 70 with apair of sides each extending along an optical axis of the waveguideelement 8 f, namely, perpendicular to the page of FIG. 3 and parallel tothe page of FIG. 4. The mesa 70 in the sides thereof are covered with aprotecting layer 71 made of iron-doped (Fe-doped) InP that shows asemi-insulating characteristic. The mesa 70 has a width of 1.5 to 3.0 μmand a height of 2.0 to 3.5 μm, each perpendicular to the optical axisthereof.

The PD element 6 d further provides two insulating films, 16 and 17,that cover and protect the protecting layer 71 and the mesa 70. Theinsulating films, 16 and 17, are made of inorganic material containingsilicon (Si), typically silicon nitride (SiN), silicon oxide (SiO₂),silicon oxy-nitride (SiON), and so on. The insulating films, 16 and 17,collectively provide an opening in a top of the mesa 70 that exposes thep-type contact layer 15 to which a p-type electrode 31 is in contact.The p-type electrode 31 may be formed by alloying a eutectic material ofgold zinc (AuZn), or AuZn with platinum (Pt) layer. Provided on thep-type electrode 31 is an interconnection 32 that extends along the mesa70 and connects the p-type electrode 31 with the signal pad 21 d. Theinterconnection 32 is formed by stacked metals of titanium-tungsten(TiW) and gold (Au), TiW/Au, or, in an alternative, stacked metals oftitanium (Ti), platinum (Pt), and gold (Au), Ti/Pt/Au; while, the signalpad 21 d may be formed by plating gold (Au) on the interconnection 32.

The insulating films, 16 and 17, further provides another opening on theconducting layer 11 in a region adjacent to the mesa 70, through whichthe n-type electrode 41 is in contact with the conducting layer 11 as acathode of the PD element 6 d. The n-type electrode 41 is physicallyapart from the buffer layer 12, and may be formed by alloying eutecticmetal of gold germanium (AuGe), or AuGe containing nickel (Ni). Providedon the n-type electrode 41 is a bias interconnection 42 that extends toa portion beneath the capacitor element 7 d to become the lower metallayer thereof. Thus, the bias interconnection 42 connects the n-typeelectrode 41 with the capacitor element 42.

Next, the waveguide elements, 8 c to 8 f, in the cross section thereofwill be described. FIG. 4 includes the cross sectional structure of thewaveguide element 8 f taken in a plane parallel to the optical axisthereof. Other waveguide elements, 8 c to 8 e have the cross sectionalstructure same with that shown in FIG. 4. The waveguide element 8 fincludes the conducting layer 11 on the substrate 10 and the waveguidestructure 80 formed on the conducting layer 11 in a region E next to theregion D of the PD element 6 d. The waveguide structure 80 includes froma side of the conducting layer 11, the buffer layer 12, a core layer 81and a cladding layer 82.

The conducting layer 11, which is common to the conducting layer 11 inthe PD element 6 d extending from the PD element 6 d to the waveguideelement 8 f, operates as a lower cladding layer in the waveguide element8 f. The buffer layer 12, which is also common to the buffer layer 12 inthe PD element 6 d, may also operate as the lower cladding layer of thewaveguide element 8 f. The buffer layer 12 extends from the PD element 6d, exactly, between the conducting layer 11 and the absorption layer 13in the PD element 6 d, to the waveguide element 8 f, exactly, betweenthe conducting layer 11 and the core layer 81 of the waveguide element 8f.

The waveguide element 8 f forms a butt joint against the PD element 6 dsuch that the core layer 81 in the waveguide element 8 f is in contactwith the absorption layer 13 in the PD element 6 d, which enables anoptical coupling between the core layer 81 and the absorption layer 13.The core layer 81 may be made of material having refractive indexthereof greater than that of the conducting layer 11 and that of thebuffer layer 12, and a lattice constant matching with that of the bufferlayer 12, which is typically, InGaAsP. In an embodiment, the core layer81 is made of InGaAsP with the bandgap wavelength of 1.05 μm and athickness of 0.3 to 0.5 μm.

The cladding layer 82 may be made of material with refractive indexsmaller than that of the core layer 81 and a lattice constant thereofmatching with the core layer 81, that is, the cladding layer 82 istypically made of InP with a thickness of 1.0 to 3.0 μm such that thecladding layer 82 in a top thereof is leveled with a top of the p-typecontact layer 15 in the PD element 6 d. The conducting layer 11 in aportion thereof, the buffer layer 12, the core layer 81, and thecladding layer 82 in the waveguide element 8 f make a mesa covered withthe insulating films, 16 and 17. This mesa surrounded with theinsulating films, 16 and 17, and the layer structure from the conductinglayer 11 to the cladding layer 82 in the mesa may effectively confinethe signal components, L_(C1) to L_(C4), to provide those signalcomponents, L_(C1) to L_(C4), to the PD elements, 6 a to 6 d.

Table below compares a structure, in particular, physical dimensionsthereof in a unit of micron-meter (μm), of an optical coupler 5 for theC-band, namely, 1528 to 1565 nm, with that for the L-band, where PSXcorresponds to a maximum space between the waveguide elements, 8 g and 8h, in the phase shifter 8 i.

C-band L-band difference W₁  22 ± 1 22 ± +1 0 L₁ 340.3 ± 0.1 330.7 ±0.1  −9.6 W₂  6.7 ± 0.1 6.7 ± 0.1 0 L₂ 186 ± 1 179 ± 1  −7 PSX  1.99 ±0.01 2.02 ± 0.01 +0.03

The optical coupler 5 for the C-band is generally designed so as tomaximize transmittance of the signal components, L_(C1) to L_(C4), andto minimize a deviation in output power between the two signalcomponents, L_(C1) and L_(C2), of the in-phase component and a deviationin the output power between the two signal components, L_(C3) andL_(C4), of the quadrature component around the center wavelength of1550±10 nm. An optical coupler applicable to the L-band is alsogenerally designed so as to maximize the transmittance of the signalcomponents, L_(C1) to L_(C4), and to minimize the deviation in theoutput power between the two signal components, L_(C1) and L_(C2), ofthe in-phase component and that between the signal components, L_(C3)and L_(C4), of the quadrature component. The optical coupler shown inthe table above equalizes the widths, W₁ and W₂, of the MMI couplers, 51and 52, but the lengths and the PSXs are discriminated. The widths, W₁and W₂, are at least one digit smaller than the lengths, L₁ and L₂;accordingly, two types of the optical couplers each applicable to theC-band and the L-band become insensitive to instability of a process offorming the optical coupler. Specifically, an optical coupler for theL-band shortens the lengths L₁ by 9.6 μm and L₂ by 7.0 μm, but widensthe PSX by 0.03 μm.

FIG. 6 shows the deviations in the output power of the optical couplerbetween the in-phase components, L_(C1) and L_(C2), denoted by a behaverG11, and between the quadrature components, L_(C3) and L_(C4), denotedby another behavior G12, respectively, where the optical coupler 5 isdesigned for the L-band. Contrary, FIG. 7 shows the deviations in theoutput power between the in-phase components, L_(C1) and L_(C2), by abehavior G11 and between the quadrature components, L_(C2) and L_(C4),by another behavior G12, respectively, for the optical coupler designedin the C-band. In FIG. 6 and FIG. 7, the horizontal axis corresponds tothe wavelength, while, the vertical axis shows the deviation. As shownin FIG. 6, the optical coupler for the L-band suppresses the deviationless than ±0.2 dB in the output power thereof between both the in-phasecomponents and between the quadrature components in the L-band. Also, asshown in FIG. 7, the optical coupler for the C-band may suppress thedeviations both between the in-phase components and between thequadrature components in the C-band.

As shown in FIG. 4, the absorption layer 13 in the PD element 6 ddirectly couples with the core layer 81 in the waveguide element 8 f,which means that the optical sensitivity of the PD device 2A may bedetermined by transmittance in the optical coupler 5, that of thewaveguide elements, 8 a to 8 f, and the sensitivity of the PD elements,6 a to 6 d. Because the L-band is closer to the bandgap wavelength ofIn_(x)Ga_(1−x)As (0<x<1) lattice matching with InP, which is 1650 nm;the absorption layer 13 in the PD element, 6 a to 6 d, inherently showsa large temperature dependence in an absorption co-efficient thereof.Moreover, when an ambient temperature lowers, the bandgap wavelength ofthe absorption layer 13 further shifts toward a shorter wavelength,namely, further closer to the L-band, which means that the sensitivityof the PD devices, 6 a to 6 d, show tendency of insufficient for the PDdevice 2A. FIG. 8 schematically shows the sensitivity of the PDelements, 6 a to 6 d, against the wavelength, where the horizontal axiscorresponds to the wavelength, and the vertical axis corresponds to thesensitivity. Because the bandgap wavelength of the absorption layer 13locates closer to the maximum wavelength in the L-band, exactly, 1650 nmin the present embodiment; the sensitivity degrades in wavelengthscloser to the longest wavelength in the L-band. Moreover, as thetemperature lowers, which shifts the bandgap wavelength of theabsorption layer in shorter wavelengths, the decrement of thesensitivity definitely becomes a subject.

The PD elements, 6 a to 6 d, for the L-band according to the presentembodiment extend the length L₃ thereof along the optical axis of thewaveguide elements, 8 c to 8 f, compared with a PD element designed forthe C-band. Specifically, the length L₃ of the PD elements, 6 a to 6 d,are set to be greater than 12 μm, while, that of a PD element for theC-band is set to be 9 μm. FIG. 9 and FIG. 10 show power distribution inthe absorption layer 13 of a PD element for the C-band and the L-band,respectively, where the power distribution is measured from an interfaceagainst the core layer 81, which is the left end in the horizontal axis,and normalized by a value at the interface. In FIG. 9 and FIG. 10, abehavior G21 corresponds to the distribution at a temperature of 25° C.,while, another behavior G22 corresponds to the distribution at atemperature of −40° C. The difference between two behaviors, G21 andG22, becomes smaller for the PD element designed in the C-band shown inFIG. 9 because the C-band is relatively apart from the bandgapwavelength of the absorption layer 13. Contrary, another PD element forthe L-band shown in FIG. 10 causes a larger difference reflecting thegreater temperature dependence of the absorption co-efficient, as shownin FIG. 10. The absorption co-efficient of the absorption layer 13 in aPD element for the C-band becomes small at a lower temperature, whichleaves the substantial distribution denoted by the behavior G22 in FIG.10 even at a region apart from the interface against the waveguideelement.

A PD element for the C-band leaves the normalized power distribution of0.1 at a point apart from the interface by 9 μm at temperatures of both25° C. and −45° C., as shown in FIG. 9. On the other hand, for anotherPD element for the L-band, as shown in FIG. 10, the normalized powerdistribution within in the absorption layer at a point apart from theinterface by 9 μm becomes 0.14 at a temperature of 25° C., but becomesabout 0.1 at the temperature of −40° C. However, the normalized powerdistribution is left around 0.1 at −40° C. even at a point apart fromthe interface by 12 μm. That is, in order to realize the sensitivity,which is comparable to that realized in a PD element for the C-band, bya PD element for the L-band, the PD element for the L-band is necessaryto extend the length of the absorption layer 13 along the optical axisof the waveguide element more than 30% compared with that of the PDelement for the C-band. However, a lengthened absorption layer in a PDelement results in an increase of parasitic capacitance attributed to ap-n junction within the PD element. Accordingly, the PD elementapplicable to the L-band preferably has the absorption layer 13 with alength L3 shorter than 19 μm.

FIG. 11 shows the sensitivity of the PD elements, 6 a to 6 d, accordingto the present embodiment, where the PD elements, 6 a to 6 d, have theabsorption layer 13 with the length L₃ of 12 μm. In FIG. 11, behaviors,G31 and G32, correspond to the sensitivity measured at temperatures of25° C. and −5° C., respectively. The sensitivity in the vertical axis isnormalized by a value at the wavelength of 1.615 μm. As shown in FIG.11, the normalized sensitivity at the respective temperatures becomescloser not only in the C-band but in the L-band.

Thus, the PD device 2A having PD elements with the type of the waveguideprovides the absorption layer 13 having the length thereof greater than12 μm along the optical axis thereof, which may compensate the decrementin the optical sensitivity at a low temperature, and the PD elements, 6a to 6 d, implemented in the PD device 2A have enough sensitivity forthe signal components in the L-band. Also, the absorption layer 13primarily includes In_(x)Ga_(1−x)As (0<x<1) with a thickness of 100 to200 nm. A thicker absorption layer in a PD element degrades thefrequency response thereof because a carrier transporting time withinthe absorption layer, in particular, the transporting time of the p-typecarriers, increases. Thus, a thinner absorption layer is preferable fora PD element in the PD device 2A. However, a thinner absorption layerdecrease absolute amount of the absorption, which results in degradationof the sensitivity of the PD element. The absorption layer 13 of thepresent embodiment provides the prolonged length along the optical axisthereof, which secures enough amount of the absorption. Accordingly, thePD element of the embodiment having the prolonged absorption layer 13may enhance the frequency response.

The frequency response of the PD elements, 6 a to 6 d, depends on a timeconstant determined by the junction capacitance and the seriesresistance, and the carrier transporting time in the absorption layer13. A thicker absorption layer reduces the junction capacitance butincreases the carrier transporting time. Contrary, a thinner absorptionlayer may shorten the carrier transporting time but increases thejunction capacitance, which increase the time constant of the absorptionlayer. That is, the optical sensitivity and the frequency response havea trade-off relation.

The PD elements, 6 a to 6 d, of the present embodiment provides thebuffer layer 12, which is the n-type conduction with the impuritydensity less than that of the conducting layer 11, or the i-typeunintentionally doped, between the conducting layer 11 and theabsorption layer 13 and between the conducting layer 11 and the corelayer 81 in the waveguide elements, 8 c to 8 f, continuous to the PDelements, 6 a to 6 d. This arrangement including the buffer layer 12 mayexpand the depletion layer at a reverse bias from the absorption layer13 to the buffer layer 12, which reduces the junction capacitancecompared with another arrangement without any buffer layer and reducesthe time constant of the absorption layer 13, which is shown in FIG. 14.That is, even when the absorption layer 13 is thinned to shorten thecarrier transporting time, the PD elements, 6 a to 6 d, may suppress theincrease of the capacitance, namely, the increase of the time constantthereof.

The buffer layer 12 may exist in the waveguide element 8 f, exactly,between the conducting layer 11 and the core layer 81. The conductinglayer 11 is highly doped with the n-type impurities, and thoseimpurities possibly become scattering centers for photons propagating inthe core layer 81, which increases the optical loss in the waveguidestructure 80. The buffer layer 12 provided between the conducting layer11 and the core layer 81 may suppress the possibility of the scatteringcross section by the impurities in the conducting layer 11.

The buffer layer 12 may have the doping density less than 1×10¹⁶ cm⁻³ toenough expand the depletion region at a reverse bias. The buffer layer12 may have the bandgap energy greater than that of the absorption layer13 but equal to or smaller than that of the conducting layer 11 toeffectively confine the signal components within the core layer 81. Thewaveguide structure 80 may optically couple with an optical fibercarrying the optical signal whose wavelength is within the range of 1565to 1612 nm, namely, in the L-band.

First Modification

FIG. 12 is a cross sectional view of another PD device 2B that includesanother PD element 6 d and another waveguide element 8 f, where FIG. 12is taken along the line IV-IV indicated in FIG. 4. Both elements, 6 dand 8 f, are modified from the PD element 6 d and the waveguide element8 f shown in FIG. 4. The arrangement of the PD device 2B shown in FIG.12 has a feature that the PD device 2B omits the buffer layer 12 commonto the PD element 6 d and the waveguide element 8 f but further providesan intermediate layer 18 in the PD element 6 d. Other arrangements aresubstantially same with those shown in FIG. 4.

The PD element 6 d includes, on the substrate 10, the conducting layer11 and the PD structure 19B formed on the conducting layer 11 in theregion D. The PD structure 19B provides stacked layers of, from the sideof the conducting layer 11, the intermediate layer 18, the absorptionlayer 13, the p-type cladding layer 14, and the contact layer 15. Thearrangements of those layers except for the intermediate layer 18including dimensions and doping conditions are same with those explainedin FIG. 4. Also, the absorption layer 13 preferably has the length L₃ atleast greater than 12 μm along the optical axis thereof.

The intermediate layer 18 may be an undoped layer, or slightly dopedwith n-type impurities by density less than that of the conducting layer11 and has bandgap energy greater than that of the absorption layer 13but smaller than that of the conducting layer 11. The intermediate layer18 of the present embodiment is made of InGaAsP doped with silicon (Si)by density less than 1×10¹⁶ cm⁻³ and having a bandgap wavelength of 1.4μm. The intermediate layer 18 preferably has a thickness of 0.1 to 0.2μm.

FIG. 12 indicates a height H₁ of the absorption layer 13 measured from atop 11 a of the conducting layer 11 to the middle of the absorptionlayer 13; that is to a virtual plane with even spans against a top 13 aand a bottom 13 b of the absorption layer 13. The intermediate layer 18may show a function to moderate discrepancy (ΔEc) in the conduction bandbetween the conducting layer 11 and the absorption layer 13. Theintermediate layer 18 may be a graded layer in composition thereof tomoderate the bandgap discrepancy ΔEc. Specifically, the intermediatelayer 18 may include two layers each made of Si-doped InGaAsP butcompositions thereof are different from each other, where one of theSi-doped InGaAsP layer in contact with the conducting layer 11 has thebandgap wavelength of 1.1 μm, while, the other Si-doped InGaAsP layer incontact with the absorption layer 13 may have the bandgap wavelength of1.3 μm. Doping density of both layers is preferably less than 1×10¹⁶cm⁻³.

The PD element 6 d may further provide another intermediate layer alsomade of InGaAsP between the absorption layer 13 and the p-type claddinglayer 14 in order to enhance high frequency response of the PD element 6d by reducing a carrier transporting time. Also, the other intermediatelayer between the absorption layer 13 and the p-type cladding layer 14may be a graded layer in the composition thereof to moderate thediscrepancy ΔEv in the valence band therebetween. This compositiongraded layer may include two layers each made of un-doped InGaAsP, ordoped with Zn by density less than 1×10¹⁷ cm⁻³, and have bandgapwavelengths of 1.3 μm and 1.1 μm, respectively.

Also, the PD element 6 d may further provide still another layer tomoderate the bandgap discrepancy between the p-type cladding layer 14and the p-type contact layer 15. The layer may be doped with Zn bydensity greater than 1×10¹⁸ cm⁻³. The layer may be constituted by twolayers each made of Zn-doped InGaAsP and having bandgap wavelengths of1.1 μm and 1.3 μm, respectively.

The waveguide elements, 8 c to 8 f, of the present modified PD device2B, where the waveguide element 8 f is representatively explained in thefollowing specification, includes the conducting layer 11 on thesubstrate 10 and the waveguide structure 80 on the conducting layer 11in the region E neighbor to the region D. The waveguide element 8 fcouples with the PD element 6 d by the butt joint such that the corelayer 81 in the waveguide structure 80 is in direct contact with theabsorption layer 13. Also, the core layer 81 is in contact with theintermediate layer 18 by the butt joint therebetween. Thus, a bottom ofthe core layer 81 and a bottom of the intermediate layer 18 are both indirect contact with the top of the conducting layer 11.

FIG. 12 indicates a height H₂ of the core layer 81 measured from the top11 a of the conducting layer 11 to a virtual plane equally apart fromthe bottom and the top of the core layer 81. The height H₂ issubstantially equal to the height H₁ of the absorption layer 13, whichmeans that the middle of the core layer 81 is substantially leveled withthe middle of the absorption layer 13.

In the PD device 2B thus described, the absorption layer 13 has thelength L₃ greater than 12 μm that is same with the PD device 2A of theaforementioned embodiment, which may secure enough sensitivity in theL-band even at a low temperature. Also, the PD elements, 6 a to 6 d, ofthe present modification provides the intermediate layer 18 with then-type conduction by slightly doping n-type impurities, or the i-type byintentionally doping no impurities. This arrangement including theintermediate layer 18 may expand the depletion layer in the absorptionlayer 13 to the intermediate layer 18 at a reverse bias, which may thinthe absorption layer 13 to shorten the carrier transporting time withoutincreasing the capacitance attributed to the absorption layer 13. The PDelements, 6 a to 6 d, may enhance the frequency response.

The intermediate layer 18 in the present modification abuts the corelayer 81 against the conducting layer 11. When the core layer 81 and theabsorption layer 13 are arranged directly on the conducting layer 11 andthe absorption layer 13 is thinned in order to enhance the frequencyresponse; the absorption layer 13 is hard to be abutted against the corelayer 81. That is, the height H₁ of the absorption layer 13 measuredfrom the top of the conducting layer 11 is misaligned with the height H₃of the core layer 81, which degrades the optical coupling efficiencybetween two layers, 13 and 81, and the optical sensitivity of the PDelements, 6 a to 6 d. The intermediate layer 18 may equalize the twoheights, H1 and H3, even when the absorption layer 13 is formed thin andmay suppress the degradation in the optical coupling efficiency betweentwo layers, 13 and 81.

Also, the stacking structure sandwiching the absorption layer 13 may besymmetrical with respect to the absorption layer 13. Semiconductorlayers, 14 and 15, showing the p-type conduction and provided above theabsorption layer 13, which are made of p-type InP and p-type InGaAsP,respectively, inherently shows a greater optical loss compared withn-type materials for the semiconductor layers, 11 and 18 under theabsorption layer 13, namely, the n-type conducting layer 11 and theintermediate layer 18 because of greater absorption of free carriers inthe p-type materials. The intermediate layer 18 with lessor carrierconcentration may moderate a difference in the refractive index betweenthe conducting layer 11 and the absorption layer 13 at the interfacetherebetween leaving the difference in the refractive index between theabsorption layer 13 and the p-type cladding layer 14 comparably greater.Accordingly, the optical confinement function against the p-typecladding layer 14 may be reduced, which suppresses the optical loss inthe absorption layer 13.

The intermediate layer 18 may have the doping density less than 1×10¹⁶cm⁻³, which may expand the depletion region of the absorption layer 13to the intermediate layer 18 when the PD element is reversely biased.Also, the intermediate layer 18 in the bandgap energy thereof may begreater than that of the absorption layer 13 but smaller than or equalto that of the conducting layer 11 to expand the depletion region in theabsorption layer to at a reverse bias to the intermediate layer 18.

Also, the conducting layer 11 may have a composition different from thecomposition of the intermediate layer 18, which may secure etchingratios for the intermediate layer 18 and the conducting layer 11 to formthe mesa 70 in the PD elements, 6 a to 6 d. That is, the conductinglayer 11 may operate as an etch stopping layer for forming the mesa 70,which may align the top 11 a of the conducting layer 11 in the region Dfor the PD elements, 6 a to 6 d, with the top 11 a of the conductinglayer 11 in the region E for the waveguide elements, 8 c to 8 f; thatis, the height H₁ in the PD elements, 6 a to 6 d, may be leveled withthe height H₂ in the waveguide elements, 8 c to 8 f.

FIG. 14 is a cross sectional view of a conventional PD device 100including a PD element 106 and a waveguide element 108, where the PDelement 106 stacks the conducting layer 11 on the substrate 10 in bothof the region D for the PD element 106 and the region E for thewaveguide element 108 but omits the buffer layer 12 and the intermediatelayer 18. Under such an arrangement, when the absorption layer 13 isformed thin for enhancing high frequency response of the absorption, theheight H₁ of the absorption layer 13 measured from the top of theconducting layer 11 causes misalignment with the height H₃ of the corelayer 81 measured from the top 11 a of the conducting layer 11, whichdegrades the optical coupling efficiency between two layers, 13 and 81.

Second Modification

FIG. 13 is a cross sectional view of another PD device 2C also modifiedfrom that shown in FIG. 4, where the cross section shown in FIG. 13 istaken along the line IV-IV indicated in FIG. 1. The PD device 2C has afeature of further implementing the intermediate layer 18 withoutomitting the buffer layer 12. Other arrangements of the PD device 2C aresubstantially same with those, 2A and 2B, of the aforementionedembodiment.

In the arrangement of the PD device 2C shown in FIG. 13, a depletionlayer at a reversed bias expands from the absorption layer 13 to thebuffer layer 12 exceeding the intermediate layer 18, which may enable tofurther thin the absorption layer 13 without increasing the junctioncapacitance, or parasitic capacitance between the electrodes. A thinnedabsorption layer 13 may shorten the carrier transporting time within theabsorption layer 13, which may enhance the response of the PD elements,6 a to 6 d, in higher frequencies. The present modification also extendsthe buffer layer 12 between the core layer 81 and the conducting layer11 in the waveguide element 8 f, which may suppress optical loss in thewaveguide element 8 f.

While particular embodiments of the present invention have beendescribed herein for purposes of illustration, many modifications andchanges will become apparent to those skilled in the art. For instance,the core layer in the waveguide element in compositions thereof is notrestricted to InGaAsP but the core layer may be made of AlInGaAsP. Also,the photodiode device in the embodiments monolithically integrates, onthe substrate 10, the waveguide elements, 8 a to 8 f, and the PDelements, 6 a to 6 d. However, the substrate 10 may integrate otherelectronic devices that convert optical signals into electrical signals,such as those including a hetero-bipolar transistor made of InP andmaterials grouped in InP, capacitors, and resistors. Also, theembodiment provides the conducting layer 11 on the substrate 10, but,the PD devices, 2A to 2C, may omit the conducting layer 11 when thesubstrate 10 is highly doped with n-type impurities to operate as aconducting layer. Accordingly, the appended claims are intended toencompass all such modifications and changes as fall within the truespirit and scope of this invention.

What is claimed is:
 1. A photodiode device, comprising: a substrateproviding a conducting layer doped with n-type impurities, theconducting layer having a first region and a second region next to thefirst region; a waveguide element provided on the first region of theconducting layer, the waveguide element including: a core layer providedon the conducting layer, and a cladding layer provided on the corelayer; and a photodiode element provided on the second region of theconducting layer, the photodiode element being optically coupled withthe waveguide element and including: an absorption layer that isprovided on the conducting layer and has a thickness thinner than athickness of the core layer, the absorption layer abutting against thecore layer in the waveguide element, and a p-type cladding layer that isprovided on the absorption layer and doped with p-type impurities, anintermediate layer provided between the conducting layer and theabsorption layer, the intermediate layer having a thickness less thandifference between the absorption layer and the core layer.
 2. Thephotodiode device according to claim 1, further including a buffer layerthat is provided on the conducting layer and extends from the firstregion to the second region, wherein the core layer in the waveguideelement is provided on the buffer layer, and the intermediate layer inthe photodiode element is provided on the buffer layer.
 3. Thephotodiode device according to claim 2, wherein the conducting layer isdoped with silicon by density at least 1×10¹⁷ cm⁻³ and the buffer layeris undoped or slightly doped with silicon by density of 1×10¹⁶ cm⁻³ atmost.
 4. The photodiode device according to claim 2, wherein the bufferlayer has a thickness 0.3 μm at most.
 5. The photodiode device accordingto claim 2, wherein the buffer layer has a bandgap wavelength shorterthan a bandgap wavelength of the absorption layer but longer than orequal to a bandgap wavelength of the conducting layer.
 6. The photodiodedevice according to claim 2, wherein the photodiode element furtherincludes the intermediate layer provided on the buffer layer in thesecond region of the conducting layer, the absorption layer beingprovided on the intermediate layer but the core layer being provided onthe buffer layer.
 7. The photodiode device according to claim 1, whereinthe intermediate layer is undoped or slightly doped with n-typeimpurities by density of 1×10¹⁶ cm⁻³ at most.
 8. The photodiode deviceaccording to claim 1, wherein the absorption layer has a height measuredfrom a top of the conducting layer to a middle between a top and abottom of the absorption layer, where the height of the absorption layeris substantially equal to a height of the core layer measured from thetop of the conducting layer to a middle between a top and a bottom ofthe core layer.
 9. The photodiode device according to claim 1, whereinthe intermediate layer include an upper layer and a lower layer, theupper layer being in contact with the absorption layer, the lower layerbeing in contact with the conducting layer, and wherein the upper layerhas a bandgap wavelength shorter than a bandgap wavelength of theabsorption layer, the lower layer has a bandgap wavelength shorter thanthe bandgap wavelength of the upper layer but longer than a bandgapwavelength of the conducting layer.
 10. The photodiode device accordingto claim 2, wherein the absorption layer is made of In_(x)Ga_(1−x)As(0<x<1) and has a thickness of 0.1 to 0.4 μm.
 11. A photodiode devicethat receives an optical signal and a local signal each havingwavelengths substantially equal to each other, the optical signalcontaining two signal components each having phases different by 90°,the optical device comprising: a substrate providing a conducting layerthereon, the conducting layer being doped with n-type impurities bydensity greater than 1×10¹⁷ cm⁻³; an optical hybrid that receives theoptical signal and the local signal, extracts the two signal componentsby performing interference between the optical signal and the localsignal, and outputs the two signal components; two photodiode elementseach having absorption layers provided on the conducting layer andp-type cladding layers on the absorption layers; and two waveguideelements that provide the two signal components extracted by the opticalhybrid to the two photodiode elements, respectively, the two waveguideelements each having core layers on the conducting layer and claddinglayers on the core layers, wherein each of the two photodiode elementshas an intermediate layer provided between the conducting layer and theabsorption layer, and the intermediate layer has a thickness less thandifference between the absorption layer and the core layer.
 12. Thephotodiode device according to claim 11, further comprising a bufferlayer that is provided on the conducting layer, wherein the core layersin the waveguide elements are each provided on the buffer layer, and theabsorption layers in the photodiode elements are each provided on thebuffer layer.
 13. The photodiode device according to claim 12, whereinthe buffer layer is undoped or slightly doped with n-type impurities bydensity of 1×10¹⁶ cm⁻³ at most.
 14. The photodiode device according toclaim 12, wherein the buffer layer has a thickness 0.3 μm at most. 15.The photodiode device according to claim 12, wherein the buffer layerhas a bandgap wavelength shorter than a bandgap wavelength of theabsorption layers but longer than or equal to a bandgap wavelength ofthe conducting layer.